NICER / ISS Science Nugget
for April 2, 2026

Prompt distribution of cross-calibration results

Every year, the International Astronomical Consortium for High-Energy Calibration (IACHEC) undertakes a coordinated observing campaign of the quasar 3C 273 to assess the relative calibrations of modern X-ray observatories. Many observatories have taken part in these observations over the past six years, including NICER, NASA's Chandra, NuSTAR, XRISM, IXPE, and Swift-XRT, as well as ESA's XMM-Newton. 3C 273 is variable, so these coordinated observations are made as nearly simultaneously as observing constraints allow. While these data are collected and typically analyzed independently by the individual instrument teams, they are most useful for the broader astrophysical community, particularly for those analyzing data from multiple observatories, when they are directly compared against one another. Unfortunately, cross-comparison results from these calibration campaigns do not quickly get published; for instance, the most recent paper on the IACHEC data for 3C 273 was published in 2017 (K. Madsen et al.). To advance our understanding of instrument cross-calibrations and to make results rapidly available to end-users, the NICER team has spearheaded an effort, led by J. Hare (Catholic U./NASA GSFC) to produce analysis pipelines for each observatory and to quickly analyze data shortly after observations are completed. The analysis results, including the measured X-ray fluxes and photon indices (parameters of a power-law model fit to each acquired X-ray spectrum), are to be published on a dedicated website hosted by NASA's HEASARC data archive, together with an accompanying paper that describes the analysis pipelines. To date, all but one of the observatories listed above have complete pipelines; the last, for XRISM, is in work.

The HEASARC webpage will enable rapid dissemination of cross-calibration results to the broader community. The eventual goal is to make the analysis pipelines themselves public, so that users can reproduce the cross-calibration results and make any changes that may be useful for their specific analyses (e.g., different energy bands, other models). This effort can also be expanded to other sources that are commonly observed for cross-calibration purposes (e.g., the Crab Nebula and pulsar).